The present invention relates to the structure of detection coils used in a magnetic sensor employing superconducting quantum interference devices (SQUIDs) which are applied to high-sensitivity magnetic sensors.
SQUIDs have been heretofore applied to detection of weak magnetic fields. Bobbin-shaped detection coils composed of superconducting wires are often used in SQUID fluxmeters.
FIG. 10 shows conventional first-derivative detection coils of the bobbin type. Shown are a bobbin 11, a superconducting coil wire 12, electrodes 13, and a damping resistor 14. The superconducting coil wire 12 is so arranged in a groove formed in the bobbin 11 that upper and lower coils are wound in opposite senses. The superconducting coil wire 12 is connected with the electrodes 13 which are connected with the damping resistor 14.
FIG. 11 shows the structure of conventional second-derivative detection coils of the bobbin type. Upper and lower coils are wound in the same sense. An intermediate coil is wound in the opposite sense with twice as many turns as the number of turns of the upper and lower coils. The wires of the first-derivative and second-derivative detection coils of the bobbin type are strands except for the circular portion of the coils so that magnetic flux is prevented from intersecting the wires. The first-derivative detection coil of the bobbin type detects the difference between the magnetic field signal detected by the upper coil and the magnetic field signal detected by the lower coil, i.e., the magnetic gradient is detected. The second-derivative detection coil of the bobbin type detects the difference between the magnetic field gradient detected by the upper and intermediate coils and the magnetic field gradient detected by the intermediate and lower coils. Therefore, the accuracy of the areas of the coils, the shape accuracy, the accuracy of the positional relation between them, the accuracy of the degree of the parallelism between them, and other factors affect the magnetic gradient sensitivity.
In the above-described conventional first-derivative detection coils of the bobbin type, the groove in the bobbin which accommodates the superconducting coil wire is required to be machined with high accuracy. Also, when the superconducting coil wire is laid, a triangular gap is formed at the boundary between each circular coil portion and strand. This makes it difficult to improve the accuracy of the areas of the coils, the shape accuracy, the accuracy of the positional relation between them, and the accuracy of the degree of the parallelism between them. As a result, the magnetic field gradient sensitivity and the reproducibility of the characteristics are deteriorated. Furthermore, the coils are difficult to manufacture, because high skill is needed for the manufacture. In addition, when the damping resistor is connected in parallel with the detection coils, it is difficult to connect the resistor directly to the detection coils. Therefore, the resistor has been connected close to the locations in which the coils are connected with the SQUID. Hence, the efficiency of the conventional devices has been low.